Add concept exercise for booleans

[mpizenberg]

Teaches the concept of booleans, with an exercise based on Annalyn's infiltration story + one function added for if-then-else expressions.

I'll still need to update the global config.json and the requirements for the concept exercise when we have merged the fundamentals exercise.

[ceddlyburge]

Hi Matthieu, this looks good to me, based on my (still coalescing) knowledge of how the concepts / exercises should be put together. Its good that the example wouldn't be obviously better represented as a Type, instead of a boolean.

[mpizenberg]

I've updated this to the latest specs and now references correctly basics-2 in requirements. This should be good to merge I think.